The present invention generally relates to a speed controller and more particularly, to a wheel speed control arrangement in an anti-lock braking system for avoiding locking of wheels during braking of a vehicle such as a motor vehicle or the like.
In a conventional anti-lock braking system as referred to above, it is generally so arranged that excessive slippage of wheels is detected through comparison between a value of a control variable calculated from a slip speed of each wheel or a differential value thereof and a predetermined threshold value, and the locking is avoided through control of the wheel speed by increasing or decreasing the brake hydraulic pressure based on the result of the comparison, while, with respect to the wheels recovered from the sign of locking, pressure is applied at a fixed rate possessed by a brake hydraulic pressure control actuator.
In the practice for effecting the anti-lock control through comparison between the control variable based on the wheel slip speed and differential value thereof and the predetermined threshold value as in the conventional anti-lock braking system described above, although the sign of locking may be effectively detected, it is difficult to detect the state of excessively small slip speed which occurs due to insufficient braking force under a state where the wheel speed becomes generally equal to the vehicle body speed, after recovery of the wheel from the sign of locking. Therefore, in order to preliminarily prevent occurrence of such inconveniences as vibrations of the vehicle body or prolonged braking distance due to falling of the wheel speed reduction below a predetermined value, which may take place as a result of the above disadvantage, it has been a conventional practice to set the pressure increasing rate of the brake hydraulic pressure control actuator to be rather large.
However, when the pressure increasing rate for the brake hydraulic pressure control actuator is set to be rather large as described above, even if the pressure reduction is effected to avoid the sign of locking upon application of a quick braking on a slippery road surface, such sign of locking soon appears again due to the large pressure increasing rate, and consequently, the control cycle is extremely shortened so as to frequently repeat the pressure increase and pressure reduction, thus resulting in uncomfortable feeling of the riding of the vehicle.